1. Field of the Invention
This invention relates to radar systems, and more particularly to methods of range correction in radar systems.
2. Description of Related Art
Radio detecting and ranging, commonly referred to as "radar", is used for detecting and locating an object of interest, or "target", using the transmission, reflection, and reception of radio waves. Radar emits radio waves in a pattern emanating from the surface of the radar's antenna. Typically, radar systems are mounted to a platform such as a tower, airplane, ship, automobile or other motorized vehicle. The objective of these radar systems is to accurately locate the position of an object of interest or target relative to the radar's platform.
A number of radar techniques are well known in the art. Radar systems have been used to determine range, angular position, and range rate of objects of interest. Target range and angular position are determined by analyzing certain properties of the return radio wave signal. Target range rate is determined by taking advantage of the well-known Doppler effect. One distinguishing feature of radar systems is the type of modulation technique used to obtain range and range rate data. Examples of these different radar systems include unmodulated continuous wave (CW) radar, frequency modulated (FM) radar, pulse Doppler radar and frequency shift keying (FSK) radar. Other distinguishing features include differences in antenna types and in the approach used in extracting angular information about a target.
Radar locates a target's position by obtaining the target's "azimuth angle" and "range" relative to a reference line or a reference point of the radar antenna. A target's azimuth angle is defined as the angular distance between the antenna reference line and a line extending from the radar antenna to the target. A target's range is defined as the distance from the antenna reference point to the target. Thus defined, a target's azimuth angle and range yield a calculated target position. Many radar systems analyze frequency domain data from the return signal to calculate the azimuth angle and range of a target's position. However, the calculated range does not always correlate exactly with the actual range. Rather, due to ambient temperature variations, oscillator voltage fluctuations, and other well-known causes, errors will occur in calculated range.
Typically, the percent range error, defined as the percent difference between calculated range and actual range, is between 10% and 30%. Unless these errors are compensated for by the radar system, inaccuracies can result in the calculation of target positions relative to the radar system platform. Therefore, it is essential that range errors are accurately estimated and calibrated by the radar system to determine a target's position precisely.
Radar has been used in a wide variety of platforms to detect the position of objects. For example, radar has been mounted on "host" automobiles and other host vehicles to detect the position of objects (such as other vehicles) on a road. One such vehicular radar system is described in U.S. Pat. No. 5,302,956, issued on Apr. 12, 1994 to Asbury et al. and assigned to the owner of the present invention, which is hereby incorporated by reference for its teachings of vehicular radar systems. Another exemplary vehicular radar system using a "monopulse" azimuth radar for automotive vehicle tracking is described in U.S. Pat. No. 5,402,129, issued on Mar. 28, 1995 to Gellner et al. and assigned to the owner of the present invention, which is also hereby incorporated by reference for its teachings of vehicular radar systems. As described therein, object position data has been used in the prior art collision avoidance systems to brake or steer a host vehicle when the radar system detects a potential collision with another vehicle. Alternatively, the radar system may be used in an intelligent cruise control system to decelerate the host vehicle when the radar system detects a potential collision with another vehicle and accelerate the host vehicle when the collision danger terminates.
In both the prior art collision avoidance systems and the prior art intelligent cruise control systems, an accurate calculation of object position relative to the radar platform is critical for safe system performance. Disadvantageously, due to ambient temperature variations and oscillator voltage fluctuations, heretofore it has been difficult if not impossible to accurately estimate and calibrate the range error. Consequently, the prior art vehicular radar systems disadvantageously often introduced range errors when attempting to determine the position of targets and therefore introduced undesirable and sometimes dangerous inaccuracies into the collision avoidance process. Therefore, a need exists for a method and apparatus that can accurately estimate the range error and subsequently calibrate the calculated range.
To more fully describe the problems associated with range error, consider the exemplary collision avoidance vehicular radar system shown in FIGS. 1 and 2. As shown in FIGS. 1 and 2, a collision avoidance vehicular radar system 100 is mounted on a host vehicle 12. The host vehicle 12 is shown in FIGS. 1 and 2 traveling in a direction of travel 22 on a road 30. As described in U.S. Pat. No. 5,302,956, the radar system 100 cooperates with control systems (not shown) on the host vehicle 12 in a well-known manner to prevent the collision of the host vehicle 12 with other objects on the road 30. For example, as shown in FIGS. 1 and 2, the radar system 100 aids the host vehicle 12 in avoiding collision with other vehicles 40, 50 travelling in front of the host vehicle 12 in a direction substantially parallel to the direction of travel 22 of the host vehicle 12. As described below in more detail with reference to FIG. 8, and as disclosed in detail in U.S. Pat. No. 5,302,956, the radar system 100 preferably includes a radar antenna 10 and a microprocessor or micro-controller 11 (FIG. 8). The radar antenna 10 preferably is mounted to a front bumper 13 of the host vehicle 12 such that it points in a forward direction substantially parallel to the direction of travel 22 of the host vehicle 12. The microprocessor 11 in the radar system 100 calculates the position of objects detected by the radar antenna 10 in a well-known manner as exemplified by the monopulse azimuth radar system described in U.S. Pat. No. 5,402,129.
As shown in FIGS. 1 and 2, the radar antenna 10 includes an antenna reference line 20 that is defined by a line emanating from the center of antenna 10 and perpendicular to the surface of the radar antenna 10. The radar antenna 10 locates "target" vehicles (e.g., vehicles 40 and 50) in a well-known manner by transmitting a transmission signal (radar beam) having at least two known frequencies, F.sub.1 and F.sub.2. The frequencies are separated in the frequency spectrum by some pre-defined frequency range. For example, in one typical application, the transmit frequencies are separated by 300 kHz, although other frequency deviations may be used. The radar system senses the returned transmission signal that is reflected back from the target vehicles. Azimuth angle 19 is calculated relative to the antenna reference line 20. For example, in one exemplary radar system, wherein the radar system 100 comprises a monopulse azimuth radar system (such as that described in U.S. Pat. No. 5,402,129), the radar antenna 10 transmits a transmission signal and senses the returned transmission signal that is reflected back from the target vehicles in two physically separated locations of the radar antenna 10. The radar antenna 10 of a monopulse radar system is split into two antennas (10a, 10b) that are physically separated by a few centimeters. This separation of the receive antenna 10 provides a "stereo-vision" perspective to the radar system 100. By comparing selected properties of the reflected signals from the two receive antennas, the radar system 100 calculates azimuth angles to target vehicles in front of the host vehicle 12. The azimuth angles to the target vehicles are determined relative to the antenna reference line 20.
The radar system 100 determines the closing rate (velocity relative to the host vehicle 12) of a selected target vehicle in a well-known manner. For example, a target's closing rate is determined by analyzing the well-known "Doppler frequency shift" in the signal returned from the target.
The radar antenna 10 includes an antenna reference point 21 that is defined as a point at the center of antenna 10. Range is calculated relative to the antenna reference point 21. The radar system 100 determines the range of a selected target vehicle in a well-known manner. For example, in one embodiment, the transmission signals F.sub.1 and F.sub.2 are generated using a Frequency Shift Keying (FSK) modulation scheme. The transmission signal F.sub.1 is defined as the carrier frequency and the transmission signal F.sub.2 is equal to the carrier frequency plus a deviation frequency. In one typical application, F.sub.1 is transmitted at 24.7250 GHz frequency whereas F.sub.2 is transmitted at 24.7253 GHz. The difference of 300 kHz between F.sub.1 and F.sub.2 is called frequency deviation and it is the stability of this frequency deviation, which influences the radar range accuracy. The target range is proportional to the difference between the phases of returned F.sub.1 and F.sub.2 signals and is inversely proportional to the frequency deviation. Thus, any drift in the frequency deviation will result in range errors.
As shown in FIG. 2, the radar system 100 preferably determines the location of a target vehicle 40 relative to the radar antenna 10 by calculating both an azimuth angle 19 and a range value of the target vehicle 40. The azimuth angle 19 is defined as the angular distance from the antenna reference line 20 to a target line 24 formed from the antenna reference point 21 to the target vehicle 40. The actual range ("R.sub.a ") 16 to the target vehicle 40 is defined as the distance from the antenna reference point 21 to the target vehicle 40. Ideally, the radar system 100 transmits and receives the signal frequencies without any variation in frequency deviation .DELTA.F (i.e., with a completely stable frequency deviation .DELTA.F). A variation in frequency deviation .DELTA.F is referred to hereinafter as a "frequency deviance" (i.e., a variation in F.sub.1 -F.sub.2 is referred to as a frequency deviation).
Thus, in an ideal radar system the frequency deviance should equal zero. When the frequency deviance is zero (as shown in FIG. 1), the calculated range ("R.sub.a ") 17 corresponds exactly to the actual range R.sub.a 16. However, due to ambient temperature variations, oscillator voltage fluctuations, and other causes, the frequency deviation .DELTA.F often drifts from its nominal value (e.g., in the typical system described above, it drifts from a nominal value of 300 kHz). Consequently, the frequency deviance, or the variations in .DELTA.F, often drifts to become a non-zero number (i.e., variations in .DELTA.F exist). When the frequency deviation .DELTA.F drifts from its nominal value (of 300 kHz, for example), a range error 15 ("R.sub.e ") is introduced (see FIG. 2). The range error R.sub.e is defined as the difference between the calculated range R.sub.c 17 and the actual range R.sub.a 16 at a given time. For example, if the frequency deviation's nominal value is 300 kHz and has drifted to 400 kHz due to changes in ambient temperature or other factors, the range calculations would have a relative range error of +33%. An exemplary range error R.sub.e 15 caused by frequency deviance (i.e., a drift in frequency deviation from its nominal value) is shown graphically in FIG. 3 for a target moving away from the radar.
As shown in FIGS. 2 and 3, because of errors and variations in the frequency deviation, a range error R.sub.e 15 is introduced into the target's calculated range R.sub.c 17 at each time instant, T, which leads to target miscalculations. Referring to FIG. 1, in the absence of frequency deviance-induced range errors (i.e., the ideal case wherein no frequency deviation errors exist and therefore no range error R.sub.e 15 (FIG. 2) is introduced), the radar system 100 accurately determines the position of the target vehicle 40 by calculating the actual range R.sub.c 17 and azimuth angle 19 of the target vehicle. Unfortunately, as shown in FIG. 2, frequency deviance creates the range error R.sub.e 15. Consequently, the prior art radar systems disadvantageously miscalculate the position of the target vehicle as being located in the incorrect position shown in FIG. 2 as phantom target vehicle 40' (i.e., R.sub.c is shortened to the incorrectly calculated range shown in FIG. 2).
Referring to FIG. 2, due to errors in the frequency deviation, the radar system 100 miscalculates the range of the target 40 as having a phantom calculated range R.sub.c 17. Thus, the radar system 100 dangerously identifies the target vehicle 40 as being at the position of phantom vehicle 40' having a calculated range R.sub.c 17, rather than as being at the true position of vehicle 40 having an actual range R.sub.a 16. This miscalculation creates a very dangerous situation for collision avoidance systems. False alarms are generated when the radar system 100 mistakenly determines that a target vehicle is in the host vehicle's direction of travel when, in fact, it is not. These false alarms can cause sudden braking and unnecessary steering of the host vehicle 12, which can lead to collisions with the target vehicle or other objects on the road 30.
False alarms can also create a nuisance condition for the operator of the host vehicle 12. The false alarms caused by the range error R.sub.e 15 can cause the operator of the host vehicle 12 to lose faith in the reliability of the radar system 100 and render the system ineffective for warning the operator of real threats. In addition, such false alarms are distracting and disturbing to the operator.
The range errors caused by variances in .DELTA.F (i.e., the frequency deviance, variations in the frequency deviation F.sub.1 -F.sub.2) in the radar system 100 can be corrected either electrically or mathematically. The frequency deviance can be corrected electrically by adjusting the frequency of the radar system 100. Alternatively, the frequency deviance can be corrected mathematically by accounting for the taking it into account when calculating the calculated range R.sub.c 17. However, regardless of the correction method used in determining the location of targets, it is essential to detect the presence of frequency deviation variation, or frequency deviance. Once detected, it is essential to accurately estimate the frequency deviation variation and to calibrate the radar system accordingly. To date, the prior art systems have provided no solution for the range errors that were introduced by frequency deviance-induced errors.
Accordingly, a need exists for a simple, inexpensive solution to the problem of detecting, estimating, and calibrating the range errors introduced by frequency deviation variations in a radar system. More specifically, a need exists for a method and apparatus that can detect, accurately estimate, and compensate for errors introduced by frequency deviation variations in a radar system. Such a method and apparatus should be simple to implement, inexpensive, and should work with existing radar systems. The present invention provides such a solution.